Lemurs are animals with extraordinary genetic diversity, even among populations that are close geographically. How do researchers identify new species based on genetic and morphological differences? Learn more...

Two new studies (1, 2) highlight the remarkable genetic diversity of lemurs in
Madagascar. The small nocturnal primates were the first placental mammals to
arrive on the island some 50 million years ago. They had no known predators
and little competition, which may explain the rapid expansion into at least
a hundred living species.

Two new studies (1, 2) highlight the remarkable genetic diversity of lemurs in Madagascar. The small nocturnal primates were the first placental mammals to arrive on the island some 50 million years ago.

The first article, published in the International Journal of Primatology,
describes two new species of mouse lemur, but the threat to habitats in
Madagascar is underscored by the fact that one of them may already be
extinct. Mouse lemurs are small and have spread throughout all forest
habitats in Madagascar. In the early 1990s, only two species were
recognized. Today there are twenty known species of mouse lemur, and that
number seems likely to grow.

The two new lemur species were first captured by co-author Rodin Rasoloarison,
a researcher at the University of Antananarivo in Madagascar, during trips
into the forest habitats in 2003 and 2007. Rasoloarison and colleagues used
two mitochondrial and four nuclear genes to place the specimens they
captured in the mouse lemur phylogenic tree. Those results were published in
2010 (3), but the current paper formally describes them as new species.

In a phylogenic tree, long branches reflect long periods of reproductive
isolation and genetic divergence, but that isn’t sufficient to declare two
populations to be separate species. Researchers must also link genetic
differences to morphological differences. Home ranges are also taken into
account, although two populations in the same range may be different species
if some other reproductive barrier is present. “You can’t do it on the basis
of DNA (alone). It’s the combination (of genetics, morphology, and
geographic range) that allows us to make these (decisions),” said Anne D.
Yoder, professor of biology at Duke University and director of the Duke
Lemur Center who was an author on the mouse lemur paper.

The mouse lemurs remain challenging to identify because they are nocturnal and
separate species look so much alike. To determine a new species, “You have
to do the meticulous morphological analysis and the genetic work as well,
but (the twenty species) are real. We’re not making this up,” Yoder said.

Discovering a new species is exciting, but in this case it was also
disheartening. When Rasoloarison returned to the lemur habitats last year,
he found the region inhabited by one of the newly identified species, the
Marohita mouse lemur, obliterated by deforestation. “I’m not sure the
species is extinct, but they are certainly on the edge of it. That’s a
crushing kind of conclusion,” said Yoder.

Yoder’s next project is to determine what drives the evolution of new lemur
species. The animals are so diverse that reproductive isolation is not
likely to be caused by geographical isolation alone. She believes it may
come down to mate selection. “They’re using some sensory criteria to decide
if they want to mate with another individual. It’s not vision because
they’re nocturnal. It would have to be auditory or olfactory,” said Yoder.
Her team is analyzing olfactory genes in hopes of finding variants that
track with speciation.

The second study, published in Proceedings of the National Academy of
Sciences, focused on a single lemur species known as the aye-aye.
Researchers sequenced the genomes of twelve individuals from the northern,
eastern, and western regions of Madagascar. Eastern and western aye-ayes
varied somewhat from one another, but the northern population was
particularly genetically distinct, suggesting that it had been
reproductively isolated for a long time.

To get a sense of the genetic distance between aye-aye populations,
researchers took the additional step of comparing the distance between the
northern and east-west populations to the distance between present-day human
groups. They compared 12 human DNA sequences from African agriculturalists,
Europeans, and Southeast Asians. The African and European human populations
are more genetically similar to one another than the northern and east-west
aye-aye populations, which are separated geographically by just 160 miles,
although that barrier includes high plateaus and large rivers.

Both studies highlight the extraordinary genetic diversity of lemurs and pose
challenges for conservationists seeking to maintain the animal’s rich
species diversity. This is particularly urgent since lemurs are the most
endangered mammals in the world, according to the International Union for
the Conservation of Nature.

And let’s not forget the appeal of those big eyes. “They’re just glorious
creatures,” said Yoder.